@Carl Love 

Thanks so much!

However, I still cannot get my head around the theory...

We should get Int(cos(2000*Pi*t)*cos(2*Pi*f*t),x=-infinity..infinity)

How does this integral = dirac delta at f = +/-1000?

Vhha

@Axel Vogt 

I also have a follow up Q:

When I try to solve this in Tutor, it does not allow me to input infinity in the limits. Says, "infinity should be a number".

@Axel Vogt 

Thanks so much for the prompt answer!

It works :)

I was wondering what is the difference between "j" and "I"? Because in mathematical formulas we generally use "j".

Also, I noticed that "pi" and "Pi" give same result.

Your feedback will help a great deal!

@Preben Alsholm 

Thanks so much for the clarification!!! I understnad now :)